This invention relates to a battery operating reversibly at room temperature, comprising an improved cathode, electrolyte and anode, and a method for manufacturing such a battery.
With a recent tendency to design various electric equipment in a micro-electronic form, batteries have been housed in an electric apparatus and integrated with the electronic elements and the circuit, such as in power sources for memory back-up of various electric devices. For this reason, the demand for minimizing the size, weight and thickness of batteries and the need for a battery having a high energy density have been increasing. In the field of primary batteries, a small-sized and light-weight battery such as a lithium battery has already been put to practical use, however, its application is limited. Under these circumstances, in the field of secondary batteries, a battery utilizing a nonaqueous electrolyte, which can be made smaller in size and weight, attracts public attention at present as an alternate battery to replace the conventional lead battery and nickel-cadmium battery. However, in battery utilizing a nonaqueous electrolyte, an electrode active material which satisfies practical physical properties, such as cycle characteristics and self-discharge characteristics, has not yet been found. Therefore, investigations are still being carried on in many research organizations.
In order to develop a small-sized and light-weight battery having a high energy density and a high reliability, it is necessary to examine the following problem areas (1) and (2):
(1) the electrode active material and the electrode; PA1 (2) the electrolyte. PA1 (R.sub.8 represents hydrogen or a lower ##STR5## alkyl group having 1 or more carbon atoms.) ##STR6## (R.sub.9 and R.sub.10 represent hydrogen or a lower alkyl group having 1 or more carbon atoms.) PA1 (R.sub.14, R.sub.15 and R.sub.16 represent a lower ##STR10## alkyl group or lower alkoxyl group, which has 1 or more carbon atoms, or a hydroxyl group.
As for problem area (1), the inventor examined a film type battery, that is, a battery having cell units with a thicknesses of 100 to 500 .mu.m, also called a "sheet-shaped" battery. With this kind of battery, however, the problem arose that the manufacture of metallic lithium foil having a desirable performance was somewhat difficult from a technical point of view and that therefore, the manufacturing process of such a battery became complicated. Further, in the secondary battery, the problem of lithium dendrite formation and passivation of the interface took place so that use of the metallic lithium was restricted. Therefore, investigations on alloys including lithium metals such as lithium-aluminum, lithium-lead and lithium-tin, have been actively carried out. However, during testing, the electrode cracked or broke into fine pieces due to repeated charging and discharging so that the cycle characteristic was not improved when these alloys were used, because these alloys, such as the lithium-aluminum alloy, have little strength. As an alternate way of restricting the formation of lithium dendrite, investigations on the selection of an electrolyte salt and improvement in separators have been examined. As for the separator, it has been attempted to restrict the formation of lithium dendrite by laminating non-woven fabrics made of polypropylene or glass fiber. However, a substantial solution has not yet been found.
Currently electrode active materials utilizing intercalation or a doping phenomenon of a layer compound are being specially studied in many research organizations. These materials are expected to have extremely excellent charge/discharge cycle characteristics, because a theoretically complicated chemical reaction does not occur at the time of the electrochemical reaction in charging and discharging. Use of carbon material as the electrode active material is a method which was developed, during the studies mentioned above, as a solution for problems of cycle characteristics and self-discharge characteristics of the electrode active material. Features of this carbon material are a high doping capacity, a low self-discharge rate and an excellent cycle characteristic. A special feature to mention is that it has a base-potential very close to that of metallic lithium.
On the other hand, problem area (2) is described below. A liquid electrolyte, especially prepared by dissolving an ionic compound in an organic electrolyte, has so far been used for the electrolyte of a battery utilizing electrochemical reactions and electrochemical devices other than the battery, such as electric double-layer capacitors and electrochromic elements etc. However, since there have been problems such as leakage of electrolyte from the battery and elution and evaporation of the electrode material etc. when a liquid electrolyte has been used, problems of long-term reliability and movement of electrolyte during the sealing process have remained unsolved. In order to solve these problems, that is, to improve the resistance of the solution to leakage and the long-term reliability, an ion-conductive high-molecular weight compound having a high ionic conductivity has been developed and further studied.
Ion-conductive high-molecular weight compounds currently being studied are straight-chain homo- or co-polymers, network crosslinked homo- or co-polymers or comb-shaped homo- or co-polymers having ethylene oxide as their basic unit. Generally, crystallization is avoided by making the compound in the form of a network crosslinked polymer or comb-shaped polymer in order to increase the ionic conductivity at low temperatures. Especially, the ion-conductive high-molecular weight compound using network crosslinked polymers has a high mechanical strength and has excellent ionic conductivity at low temperatures.
Electrochemical cells using such ion-conductive high-molecular weight compounds are described widely in many patent documents, such as, for example, U.S. Pat. No. 4,303,748 (1981) by Armand et al., U.S. Pat. No. 4,589,197 (1986) by North, and U.S. Pat. No. 4,547,440 (1985) by Hooper et al. A feature of these cells is the use of an ion-conductive high-molecular weight compound prepared by dissolving an ionic compound in a high-molecular weight compound having a polyether structure.
In order to use the ion-conductive high-molecular weight compound as the electrolyte of batteries utilizing electrochemical reactions and electrochemical devices other than the battery, the high-molecular weight compound must have both high ionic conductivity and high mechanical properties (mechanical strength and flexibility etc.). However, these properties contradict each other. In many patent documents described above, for example, the compound is operated at a high temperature because the ionic conductivity at a temperature lower than room temperature decreases down to below a practical range. Therefore, as a simple way to improve the ionic conductivity for example, a method is proposed, in Published Patent Application (KOKAI) No. 59-149601, Published Patent Application (KOKAI) No. 58-75779, U.S. Pat. No. 4,792,504 etc., wherein an organic solvent (preferably, an organic solvent with high permittivity) is added to the ion-conductive high-molecular weight compound to maintain a solid state. In this method, however, while the ionic conductivity is improved the mechanical strength is worsened. In the electrode active material utilizing intercalation or a doping phenomenon of the layer compound, expansion and contraction of the electrode active material are produced accompanied by charging and discharging. To cope with this problem, the mechanical strength of the electrode and the electrolyte must be improved.
When an ion-conductive high-molecular weight compound is used as the electrolyte for electrochemical devices, it becomes necessary for the electrolyte to be made in the shape of a film in order to reduce the internal resistance. Especially, this is important for film type batteries. In the case of ion-conductive high-molecular weight compounds, it is possible to work a uniform film easily into shape. Several methods for this purpose are known, for example, such as a method in which a solution of the ion-conductive high-molecular weight compound is cast and the solvent is evaporated and removed, a method in which a polymeric monomer or macromer is applied to a substrate to be heated and polymerized, or a method in which curing is done with radiation. It is possible to obtain a uniform film when these methods are used. However, short-circuiting sometimes occurs due to breakage of the electrolyte layer caused by its compression deformation when laminating the ion-conductive high-molecular weight compound in between the electrodes to assemble the battery and electrochromic element etc. Accordingly, in order to make the ion-conductive high-molecular weight compound into a uniform film, an increase of mechanical strength is important, in addition to good ionic conductivity.